Summary - Overall Patients with obstructive sleep apnea (OSA) may have hundreds of cycles over the night of loss of airway dilator motor tone and airway obstruction, followed by apnea, which is ended by an arousal, in which there is EEG desynchronization accompanied by return of airway dilator muscle tone, opening of the airway, and re- established ventilation. The EEG arousals cause sleep fragmentation and sleep loss, resulting in cognitive impairment, and metabolic and cardiovascular consequences. We hypothesize that by augmenting brain circuits that keep the airway open while suppressing the EEG arousals, we can prevent these outcomes. We have found that the EEG arousal depends on two circuits, the CGRP-expressing neurons in the parabrachial nucleus (PBCGRP cells), and the dorsal raphe serotonin neurons that provide input to them. The increase in airway dilator tone, in part through genioglossus muscle (GG) tone, allows breathing to restart in OSA, and relies on two different circuits: FoxP2 neurons in the PB (PBFoxP2 neurons) and medullary serotonin neurons that innervate the medulllary respiratory control system. Project 1 will examine the effects on ventilation and GG-EMG of activating or inhibiting the PBFoxp2 neurons optogenetically and the firing of PBFoxP2 neurons in real time with calcium imaging,.at baseline and during CO2 exposure. It will then use chemogenetics to enhance the firing of the PBFoxP2 neurons and ventilator (tidal volume, respiratory rate) and GG-EMG response, while inhibiting the PBCGRP neurons and EEG arousal during CO2 exposure. Project 2 and 3 will run in parallel to identify the forebrain inputs to the PBCGRP and PBFoxP2 neurons that activate them during EEG arousal. Their shared strategy is to identify druggable receptors on the PB cells that respond to CO2, to suggest therapies that can be used to augment firing of PBFoxP2 neurons and suppress PBCGRP neurons during CO2 exposure. They will use single cell RNA-Seq to identify the receptors on these neurons, and rabies virus tracing combined with channelrhodopsin-assisted circuit mapping to determine their inputs, and then GCaMP6 fiber photometry to determine which of these inputs is activated during the EEG arousal that accompanies CO2 exposure. Project 4 examines the inputs to the respiratory control system from the medullary serotonin neurons that are required to produce the ventilatory and GG-EMG response to CO2. It takes advantage of identifying genetically distinct subsets of medullary serotonin neurons that innervate the sensory and motor components of the respiratory control system. It will then identify the forebrain inputs to these different serotonin neurons, to determine which ones activate them, and with what receptor types, during CO2 exposure. Finally, Project 5 will use information from Projects 1-4 that identifies druggable receptors that increase airway dilator tone, while suppressing EEG arousals during sleep apnea. We expect with refinement of the receptor types that need to be stimulated or inhibited, we can design drug combinations to keep the airway open while preventing the EEG arousals that result in the long term deleterious consequences of OSA.